All dash lights are powered by a gray wire with a blue line...newer models have a violet line on the gray wire.
The W201 has a central connection point in the heater housing. It is located in the top of the heater housing at the left side and can be seen after the dash cluster is removed (it is practically behind the rear defroster switch). The wires are connected to this point by a screwed tight eye...
with a minor 600+k